1. Field of the Invention
The present invention relates to semiconductor circuit technology, and more particularly, to an analog variable gain amplifier (VGA) adjusting a signal level of a mobile communication system, and even more particularly, to design of a VGA employing an operational transconductance amplifier (OTA) having a wide linear input/output range.
2. Discussion of Related Art
In a mobile communication terminal illustrated in FIG. 1, transmission power of a transmitter should be controlled in order to avoid excessive interference with another terminal. Such power control is performed by an automatic gain control (AGC) circuit, thereby maximizing a dynamic range of the entire system. The core cell of the AGC circuit is nothing but a VGA providing a gain in proportion to a control voltage. AGC circuits are not only used in mobile communication systems but have an extensive range of applications including disk drives, hearing aids, and optical receivers.
The VGA provides a voltage gain that increases exponentially with an AGC signal controlled by a baseband processor (BBP), thereby providing a voltage gain that, in decibels (dB), is linearly proportional to a control voltage. Conventionally, a bipolar junction transistor (BJT) device having an exponential function characteristic has been mainly used for the VGA. However, owing to demand for a complementary metal-oxide-semiconductor (CMOS) system-on-chip (SoC) and development of various circuit technologies, many VGAs having a linear gain characteristic on a logarithmic scale are currently embodied using a CMOS device. However, such CMOS VGAs use many blocks to overcome characteristics of a CMOS device and obtain a linear gain characteristic on a logarithmic scale, which results in a complicated structure and excessive current consumption.